Shape memory is the ability of a material to remember its original shape, either after mechanical deformation, which is a one-way effect, or by cooling and heating, which is a two-way effect. This phenomenon is based on a structural phase transformation.
Materials known to have these properties are shape memory alloys (SMAs), for example TiNi, CuZnA1, and FeNiA1 alloys. The structure phase transformation of these materials is known as a martensitic transformation. These materials have been proposed for various applications such as vascular stents, medical guidewires, orthodontic wires, vibration dampers, pipe couplings. However, these materials have not been widely used, in part due to their relatively high costs and their limited range of mechanical properties.
Shape memory polymers (SMPs) have been under active development as a replacement or augmentation to SMAs. SMPs enjoy many advantages, among which are low density, high recoverable strain (up to several hundred percent compared to less than 8% for SMA), tailorability of the transition temperature and rubbery modulus according to the application, easy processability, and economy of materials and manufacturing. In the literature, several classes of polymers have been shown to allow SMP behavior, including highly entangled amorphous polymers, crosslinked amorphous polymers (including castable SMPs), melt-miscible blends of semicrystalline and amorphous polymers, crosslinked semicrystalline polymers and their blends with rubber (shape memory rubber), and multiblock copolymers. The latter SMP class consists of phase-segregated linear block co-polymers having a hard segment and a soft segment. The hard segment is typically crystalline, with a defined melting point, and the soft segment is typically amorphous, with a defined glass transition temperature. In some embodiments, the hard segment is amorphous and has a glass transition temperature rather than a melting point. In other embodiments, the soft segment is crystalline and has a melting point or glass transition temperature. The melting point or glass transition temperature of the soft segment is substantially lower than the melting point or the glass transition temperature of the hard segment.
When the SMP is heated above the melting point or glass transition temperature of the hard segment, the material can be shaped with complete relaxation of internal stress. This original shape can be memorized by cooling the SMP below the melting point or glass transition temperature of the hard segment. When the shaped SMP is cooled below the melting point or glass transition temperature of the soft segment while the shape is deformed, that temporary shape is fixed. The original shape is recovered by heating the material above the melting point or glass transition temperature of the soft segment but below the melting point or glass transition temperature of the hard segment. In another method for setting a temporary shape, the material is deformed at a temperature lower than the melting point or glass transition temperature of the soft segment. When the material is heated above the melting point or glass transition temperature of the soft segment, but below the melting point or glass transition temperature of the hard segment, the stresses and strains are relieved and the materials return to their original shape. The recovery of the original shape, which is induced by an increase in temperature, is called the thermal shape memory effect.
The shape memory effects are intimately linked to the polymer's structure and morphology and exist in many polymers, copolymers and cross-linked polymers. Examples of polymers used to prepare hard and soft segments of SMPs include various polyethers, polyacrylates, polyamides, polysiloxanes, polyurethanes, polyethers amides, polyurethane/ureas, polyether esters (U.S. Pat. No. 5,506,300 to Ward et al., U.S. Pat. No. 5,145,935 to Hayashi, and U.S. Pat. No. 5,665,822 to Bitler et al), polynorborene (Japanese Patent Publication No. JP 59-53528 (Nippon Zeon Co. Ltd)) cross-linked polymers such as cross-linked polyethylene and cross-linked poly(cyclooctene) (C. Liu, S. B. Chun, P. T. Mather, L. Zheng, E. H. Haley, and E. B. Coughlin, Macromolecules, volume 35, number. 27, pages 9868–9874 (2002)), inorganic-organic hybrid polymers (H. G. Leon, P. T. Mather, and T. S. Haddad, Polymer International, volume 49, number 5, pages 453–457 (2000)), and copolymers such as urethane/butadiene copolymers, styrene-butadiene copolymers (M. Irie, Chapter 9: Shape Memory Polymers, in K. Otsuka and C. M. Wayman, eds., “Shape Memory Materials,” Cambridge University Press: Cambridge, UK, 1998).
As described above, the recovery of the original shape of a SMP or SMA is triggered by the application of heat that increases the temperature of the SMP or SMA beyond the critical temperature, be it a melting point or glass transition temperature. To date, application of heat has been primarily from external sources, such as heat guns, or hot water. However, new applications of shape memory materials would be possible if the heat necessary to allow shape recovery in a shape memory material were generated within or immediately adjacent to the shape memory article itself.